1. Field of the Invention
The present invention relates to an on-demand type inkjet recording device, and more specifically a line-scanning type high-speed inkjet recording device having a plurality of nozzles.
2. Related Art
There have been proposed a continuous inkjet recording device that continuously ejects ink droplets and an on-demand inkjet recording device that ejects ink droplets only when needed.
Because the on-demand inkjet recording device ejects ink droplets only when needed, non-ink-ejection periods occur during printing operations. When a water-based ink is used in such an on-demand type inkjet recording device, the water-based ink clinging around nozzles evaporates and thus gets dense during the non-ink-ejection periods. Condensed ink prevents proper ink ejection, and in a worse case blocks off the nozzles, thereby disabling ink ejection.
Although such a problem does not occur in the continuous-type inkjet recording device, this is a serious problem in the on-demand type inkjet recording device.
In order to overcome this problem, Japanese Patent-Application Publication No. SHO-57-61576 has proposed a device that performs ink vibration for generating vibration in ink inside the nozzles by applying a driving energy smaller than that for ejecting ink to a piezoelectric element. In this manner, ink solidification is prevented, and thus clogging in the nozzles due to solidified ink is prevented. However, because the ink vibration cannot prevent evaporation of ink, if ink ejection is not performed over a long time period, then the ink will be gradually condensed, resulting in improper ink ejection or even ejection failure.
Japanese Patent-Application Publication NO. HEI-9-29996 has proposed a device that overcomes the above problem by performing ink refresh operations in addition to the ink vibrations. In the ink refresh operations, a recording head ejects refresh ink droplets to remove defective ink from the nozzles. Because the condensed ink is removed from and fresh ink is supplied to the nozzles, preferable ink ejection performance is reliably maintained.
However, this ink refresh operation cannot be performed in a printing region where the recording head is in confrontation with a recording sheet. Accordingly, when the ink refresh operation is needed during the printing operation, it is necessary to stop the printing operation and to move the recording head out of the printing region. This requires a considerable amount of time, and reduces the overall printing speed, and also wastes ink. However, decreasing the frequency of the ink refresh operations in order to accelerate the printing speed and to save the ink increases the danger of nozzle clogging due to condensed ink.
Also, there has been provided a line-scanning-type recording device that includes a recording head formed with nozzle arrays. Because the recording head has a width equivalent to the entire width of a recording sheet, printing is performed on the recording sheet that is being transported in its lengthwise direction relative to the recording head without moving the recording head in the widthwise direction across the recording sheet. With this configuration, the printing operation is performed at high speed.
In this line-scanning type recording head, however, it is difficult to stop the high-speed printing operation for the ink refresh operation. Moreover, it takes long time to move the recording head out of a printing region. Although it is conceivable to perform the ink refresh operation between pages, this is impossible when a continuous sheet rather than cutout sheets is used.
Moreover, once the printing operation is started in the high-speed inkjet recording device, such as the above mentioned line-scanning type recording device, that prints at 100 ppm (page/minute) or more, the recording device is expected to continue the printing more than ten minutes (1,000 pages or more) without stop. Accordingly, in order to satisfy this ten-minute requirement, it is necessary to maintain the proper ink ejection by the ink vibrations alone without the ink refresh operations.
However, the effect of the ink vibration on ink ejection performance lasts for only several seconds to several tens of seconds. Also, because there are usually several million of nozzles formed in a single line-scanning type recording head, it is extremely difficult to keep each of the nozzles in good ejection condition for more than ten minutes by the ink vibration only.
It is an objective of the present invention to overcome the above problems and to provide an on-demand ink jet recording device capable of maintaining its proper ink ejection without stopping printing operation.
In order to achieve the above and other objects, there is provided an inkjet recording device including an ejection means for ejecting ink droplets and a driving signal generation means for generating a print-driving signal and a maintenance signal. The ejection means ejects print ink droplets as the ink droplets when the print-driving signal is generated, and the ejection means performs maintenance operations when the maintenance signal is generated. The print ink droplets reach a recording medium to form dots on the recording medium. The print-driving signal is repeatedly generated at a predetermined time interval, and the maintenance signal is repeatedly generated at the predetermined interval in a time phase different from the print-driving signal.